The mechanism of action of several pyridoxal 5'-phosphate dependent enzymes is being studied and compared. These include tryptophanase, D- serine dehydratase, histidinol-P aminotrasferase, and ornithine decarboxylase, all from bacterial sources. The first two catalyze formally similar reactions that involve enzyme-bound alpha-aminoacrylate as an intermediate. The mechanism of decomposition of this complex, which seems to differ in the two enzymes, is being studied by borohydride reduction and other methods. Covalent modification studies of histidinol-P aminotransferase is underway to attempt to implicate specific amino acid residues of the protein moiety as part of the active site. The sequence of the pyridoxyl peptides derived by borohydride reduction of this enzyme and of ornithine decarboxylase is to be determined. Finally, we are studying the mechanism of uptake of (H3)-labeled vitamin B6 by various bacterial cells and hope to further characterize it. These projects are closely related and not always well separated from those supported by a second USPHS grant, Nutrition and Metabolism of Microorganisms (AI 01575).